Not Applicable.
1. Field of the Invention
This invention relates to the use of a magnetic field to affect microstructural changes in a metallic material, first, by relieving residual stress at ambient or cryogenic temperatures and, second, in the case of ferrous alloys, by reversing retained austenite stabilization.
2. Description of the Related Art
Metal working procedures such as casting, forging, welding, heat-treating, and forming introduce residual stresses into components. FIG. 1 is a schematic describing the evolution of residual stresses and the unfavorable effects of residual stresses in components manufactured using prior art processes. Undesirable residual stresses in components are a major issue that many industries have to deal with. For example, the issues can be distortion during machining, cracking before tempering (stress relieving thermal treatments), or accelerated corrosion while in use.
Residual stresses are non-applied stress resulting from a constrained volume change. In metals, these stresses are elastic and are typically the direct result of elastic and plastic strains due to thermal gradient and phase transformation strains in addition to crystallographic anisotropy effects. Various stress relieving processes can cause movement of dislocations and relieve residual stresses through enhancing the mobility of the dislocation structure. This can lead to their rearrangement, multiplication, or annihilation, thereby altering the residual stress profile in a sample. Abatement of tensile residual stresses is very beneficial from both a component design and life expectancy viewpoint, since existing residual stresses typically reduce the design stresses and fatigue life. Residual stress relief is known by way of heat treatment in an oven, mechanical vibration, cryogenic treatment, or laser or annealing. U.S. Pat. Nos. 6,144,544 and 4,873,605, and Wu et al. in xe2x80x9cMicromechanism of residual stress reduction by low frequency alternating magnetic field treatmentxe2x80x9d, Materials Science and Engineering A328 133-136, 2002, also describe residual stress relief through the use of pulsed magnetic fields. However, these pulsed magnetic fields are generated by resistive electromagnet systems that rely on capacitors or other circuitry to pulse the magnetic field. Thus, these resistive magnet systems use large amounts of energy such that the processes are economically unattractive.
The use of either ac or dc electric or magnetic fields under certain exposure times and field intensities has also been proposed to partially repair high cycle fatigue damage as evidenced in some extension of fatigue life (see, for example, Conrad et al., Mater. Sci. Eng. A145, 1, 1991; Zhao et al., Eng. Fract. Mech. 46, 347, 1993; Fahmy et al., Scripta Mater. 38, 1355, 1998; Bhat et al., Int. J. Fatigue, 15, 193-197, 1993; and Bao-Tong et al., Scripta Mater. 40, 767, 1999). However, certain test conditions reported resulted in an opposite effect, i.e., a reduction in fatigue life occurred. This effect was not explained. The hypothesis suggested for the life improvement conditions was that accumulated fatigue damage (before the crack initiation stage) was eliminated or reduced during application of the electric or magnetic fields. The damage relieved in the early stages of fatigue in metals is the irreversible cyclic plastic deformation. This damage is observed in TEM studies of fatigued specimens as a developing dislocation substructure. The terms electroplasticity (see, H. Conrad and A. F. Sprecher, in Dislocations in Solids, ed. F. R. N. Nabarro, vol. 8, Elsevier, Amsterdam 1989) and magnetoplasticity (see, Alshits et al., J. Alloy Compd. 211/212, 549, 1994) have been presented in prior research studies where the presence of an electric current or magnetic fields appeared to reduce the plastic deformation resistance of metals resulting in the movement of dislocations without an external load.
Steel heat-treating processes can also produce retained austenite in a workpiece. Retained austenite is deleterious in many applications because this phase can transform subsequently upon the application of an external stress promoting high carbon martensite formation. This material is very brittle and can lead to catastrophic failure in service. Also, in high performance applications, where high-speed bearings are machined to very high tolerances, if the austenite transforms under load, seizing can occur and cause major system damage as a result of a large positive phase transformation volume strain (xcx9c4%).
Various industrial processes have been proposed to minimize retained austenite. FIG. 2 is a schematic showing two prior art processes for producing steel with minimal amounts of retained austenite. As the carbon level of steels is increased ( greater than xcx9c0.5 wt. % carbon), the amount of transformation of austenite to martensite upon quenching is reduced as a result of processes that interfere with the nucleation and growth of the martensite plates that are the preferred microstructure. Typically, the martensite finish temperature, MF, is below ambient temperature for these higher carbon alloys and so some untransformed austenite will exist under ambient conditions.
When cooling is resumed after some hold time at ambient temperature, the austenite to martensite transformation does not progress as far to completion as would have been accomplished if no isothermal hold had occurred. The amount of austenite stabilization that results generally is less the closer the isothermal arrest temperature is to the MS temperature, or stated a different way, the retained austenite is less when you have less martensite at the arrest temperature. An explanation for this is the fact that formation of the martensite plates leads to accommodating plastic deformation in the surrounding matrix. This can lead to very high dislocation densities in the austenite that can interact with the glissile dislocations in the martensite plate boundary and cause the martensite interface to no longer be mobile which will inhibit further growth of the martensite plate and stabilize the retained austenite.
Dislocation densities on the order of 1011-1012 cmxe2x88x922 have been observed in these quenched steels in contrast to dislocation densities of 106-108 cmxe2x88x922 for annealed microstructures. Subsequent migration of the interstitial carbon atoms to the dislocations (i.e., pinning) will further enhance the stabilization process, which explains why the degree of austenite stabilization is observed to increase to a maximum with time. This pinning of dislocations by carbon in ferrous martensites has been confirmed through internal friction measurements that reveal a Snoek peak in the internal friction intensity versus temperature plot, which occurs as a result of stress-induced movement of carbon atoms.
The presence of retained austenite upon quenching to room temperature is a major heat treatment concern since the heat treater must quickly move the quenched components to an alternate quench facility where cryogenic treatments must be performed to drive the transformation to completion (see prior art process #1 in FIG. 2). Generally, this additional processing step still leaves a residual amount of retained austenite. Alternately, multiple tempering heat treatments are employed to successively temper the initial martensite and form martensite from the retained austenite upon further cooling that must be re-tempered to transform the new martensite to the ferrite and carbide microstructure (see prior art process #2 in FIG. 2). The elimination of these additional heat treatment steps would result in energy savings and reduced greenhouse gases. In other prior processes, it has been proposed to control austenite formation by the application of magnetic field gradients to steel (see U.S. Pat. No. 5,885,370) or by the high temperature application of a magnetic field to steel (see U.S. Pat. No. 6,375,760).
Therefore, in view of the well known advantages of stress relieving processes and the advantages associated with reducing retained austenite in steel components, there is a continuing need for alternative processes for relieving residual stress in metals and for reversing retained austenite stabilization in ferrous alloys.
The foregoing needs are met by a method according to the invention for relieving residual stresses in a component formed from a metallic material and for affecting a phase transformation in a metallic material. In a first aspect, the invention provides a method for relieving residual stresses in a component formed from a metallic material wherein the method comprises applying a constant magnetic field of at least 1 Tesla to a metallic material at a temperature between about 10xc2x0 C. and about 50xc2x0 C. The first aspect of the invention is particularly useful in relieving residual stresses in a ferrous alloy or a ferromagnetic material.
In a second aspect on the invention, there is provided a method for affecting a phase transformation in a metallic material wherein a constant magnetic field of at least 1 Tesla is applied to a metallic material at a temperature between about 10xc2x0 C. and about 50xc2x0 C. The second aspect of the invention is particularly useful in affecting a phase transformation in a ferrous alloy or a ferromagnetic material.
In a third aspect on the invention, there is provided a method for affecting transformation of retained austenite to martensite in a ferrous alloy having martensite and retained austenite wherein a constant magnetic field of at least 1 Tesla is applied to a ferrous alloy having martensite and retained austenite at a temperature between about 10xc2x0 C. and about 50xc2x0 C. In one version of the third aspect of the invention, the ferrous alloy is thereafter placed in an environment having a temperature high enough to form tempered martensite. In another version of the third aspect of the invention, the ferrous alloy is thereafter placed in an environment having a transformation temperature low enough to transform any remaining portion of the retained austenite to martensite.
It is therefore an advantage of the present invention to provide a method for relieving residual stress in metals and for reversing retained austenite stabilization in ferrous alloys.
It is another advantage of the present invention to provide a method for relieving residual stress in metals and for reversing retained austenite stabilization in ferrous alloys that impacts a significant cross section of industry including casting, forging, forming, heat treating, steel, aluminum, and the industries that make use of metallic components
It is a further advantage of the present invention to provide a method for relieving residual stress in metals and for reversing retained austenite stabilization in ferrous alloys that results in energy savings and the elimination of greenhouse gases from current heat treating processes.
It is yet another advantage of the present invention to provide a method for relieving residual stress in metals that allows for the tailoring of residual stress profiles for specific applications in fabricated components.
It is still another advantage of the present invention to provide a method for reversing retained austenite stabilization in ferrous alloys that eliminates tempering heat treatments to improve ductility by reducing the high dislocation density in quenched martensitic microstructures.
It is a still further advantage of the present invention to provide a method for relieving residual stress in metal components and for reversing retained austenite stabilization in ferrous alloys that enhances component life and performance through the elimination of undesirable residual stresses or the reduction of stabilized retained austenite that transforms to brittle martensite upon loading and reduces toughness that can lead to catastrophic failure.
It is still another advantage of the present invention to provide a method for relieving residual stress in metal components such that material and fabrication savings and reduced scrap will result through the residual stress relief because fabrication to near-net shape would become a reality for many industrial applications since residual stresses can be eliminated or significantly reduced before machining operations.